This invention relates to scanners for the acquisition of image information, and more particularly to a device and method, which may calibrate the temperature dependent video output of full width array (FWA) image sensors.
Calibration is required in document scanner applications that use image sensors to achieve a uniform document image. Individual pixel gain and offset errors must be compensated to provide a uniform sensor output. These errors are due to the effects of normal IC processing differences across the sensor, sensor drift, lamp drift and contamination. Calibration is also done to correct for image sensor dark DC level drift with temperature variation, and also to account for lamp and other component aging. During calibration, a black reference point may be collected with the illumination system turned off or by sensing a black calibration target. A white reference point may be collected in a similar manner, e.g., by sensing a white target underneath the platen outside the image area.
Calibration is most often done at start-up during a warm-up period prior to image production. This calibration period normally may take several minutes due to the necessity for components to achieve a relative stable temperature in order to maintain an acceptable level of image quality. Most calibration schemes use a simple calibration strategy having a calibration equation of:yi=mixi+bi  (1)where for every i pixel there is a gain factor, mi, and an offset factor, bi, applied to the sensor output, xi, to derive the desired sensor output yi. The values of bi may be set so that yi is same minimal value for all pixels, with no light signal, and mi values may be set so that yi has the same maximum value for all pixels with the maximum possible light signal. The values of mi and bi may be stored in a memory and used during real-time correction of sensor video output.
Mechanical devices have been developed to position an optical scanning assembly from a reference position to a calibration position so as to scan calibration target. An exemplary apparatus for physically positioning an FWA image sensor relative to a fixed calibration target for normalizing the sensor response across the array is discussed in U.S. Pat. No. 4,967,233.
U.S. Pat. No. 5,280,368 discloses an apparatus for calibrating a fixed scan head in which an arrangement for providing a calibration target is provided. Unlike the '233 patent in which the scan head moves relative to the calibration target, the '368 apparatus fixes the position of the scan head and adjusts the position of the calibration target relative to the scan head.
U.S. Pat. No. 6,038,038 further discloses a method of compensating for the offset present in image signals produced by an image processing apparatus in an image sensing device or scanner. The method utilizes intermediate reflectance targets to measure the response of each sensor element and then, using the measured responses, fits a characteristic curve to the responses to calculate the response of the sensor element.
Each of the foregoing references is incorporated herein by reference in its entirety.